Cameras, mobile phone cameras and 3D-image processing devices often utilize a zoom lens to automatically focus, enlarge or reduce an image, so as to obtain a desired image. Conventionally, a zoom lens has multiple lens groups. These lens groups move along an optical axis of the lens to thereby change the spaces among them and accordingly, the overall focal length of the lens without affecting the imaging distance thereof. However, this type of lens needs relatively long lens group movement distance, and the movement distances of the lens groups are not in a linear relationship. Therefore, the zoom lens has difficulties in the structural design and control precision thereof, and has high manufacturing cost that could not be easily lowered. Currently, a liquid lens or liquid crystal lens (LC lens) has been used to improve the movement distances of lens groups in order to obtain reduced camera size.
According to the general principle of liquid lens, the liquid lens includes a tunable liquid-filled lens and a solid lens. By changing the shape of the liquid-filled lens to be a biconvex lens or a biconcave lens, or by using filling media of different refractive indexes, it is able to tune the focal length of the lens and achieve the purpose of zooming. Or, by using the liquid crystal-filled lens and changing the refractive index of the liquid crystal with an applied electric field, an electrically tunable-focus LC lens can be realized, such as the LC lens revealed in Susumu Sato: “Liquid-crystal Lens-cells with Variable Focal Length”, Japanese Journal of Applied Physics, published on Mar. 12, 1979.
Another prior art optical element is disclosed in US2007/0139333, which includes a first substrate having a first electrode, a second substrate, a second electrode arranged outside the second substrate and having a hole, and a liquid crystal layer provided between the first and the second substrate. The first electrode and the second electrode are electrically connected to each other. By applying a voltage to the liquid crystal layer, it is able to change an orientation of liquid crystal molecules in the liquid crystal layer. Further, a dielectric layer and a third electrode are arranged on the second substrate. With the third electrode, it is able to regulate image quality and tune lens focal length. However, the optical element disclosed in US2007/0139333 has the following disadvantages:
(1) Having complicated structure to increase the manufacturing cost thereof; and
(2) Requiring high driving voltage to thereby increase the driving circuit cost and power consumption thereof.